1. Field of the Invention
The present invention relates to fractional melt flow rate polymer powders, i.e., powders of polymers which have a melt index less than 1, and which are spherical or substantially spherical in shape. The invention also relates to a process for producing the improved polymer powders having reduced melt flow rates.
2. Description of the Prior Art
The use of thermoplastic resin powders is well documented in the prior art. For example, powdered thermoplastic resins in dry form have been used to coat articles by dip coating in either a static or fluidized bed, by powder coating wherein the powder is applied by flame or electrostatic spraying or dusting. Powders can also be applied in dispersed form, by roller coating, spray coating, slush coating, and dip coating substrates such as metal, paper, paperboard, and the like. Powders are also widely employed for conventional powder molding processes, e.g., rotational molding. Still other applications for powders include use as paper pulp additives; mold release agents; additives to waxes, paints, caulks, and polishes; binders for non-woven fabrics; etc.
Besides the physical properties of the powder, which are dictated by the resin being used, the size and shade of the particles are the other major properties which influence the selection of a powder for a particular application. These latter properties are primarily a function of the process by which the powders are prepared, which can include mechanical grinding, solution processes and dispersion processes. Particle size is determined using U.S. Standard Sieves or light scattering techniques and, depending on the method used, will be reported in mesh size or microns. The inverse relationship between the sieve size (mesh number) and particle size (in microns) is well documented and conversion tables are available. The shape of the particles is ascertained from photomicrographs of the powders. Particle shape has a marked influence on the bulk density of the powder and its handling properties.
For most effective fluidization and dry spraying, it is generally considered advantageous to use powders which have a fairly narrow particle size distribution and wherein the particles are spherical in shape. Powders produced by mechanical grinding or pulverization, which are typically irregular in shape and generally have quite broad particle size distributions, are not well suited for fluidization and dry spraying. While the particles of powders produced by solution processes are less irregular than those obtained by mechanical means, they are still not spherical.
Powders obtained using dispersion techniques, such as those described in U.S. Pat. Nos. 3,422,049 and 3,746,681, wherein the particles produced are spherical in shape and fall within a relatively narrow size range are most advantageously employed for fluidization and dry spraying. These processes involve subjecting a synthetic organic polymeric thermoplastic resin in molten form and about 0.8 to 9 parts by weight of water per part of resin to vigorous agitation in the presence of from about 2 to 25 parts by weight per 100 parts of resin of a water-soluble block copolymer of ethylene oxide and propylene oxide having a molecular weight above about 3500 and containing at least about 50% by weight of ethylene oxide in the absence of an organic solvent for the polymer and at a pressure between about 6 and 120 atmospheres such that a fine dispersion is produced and then cooling said dispersion to below the softening temperature of the said resin.
A continuous process for the preparation of finely divided polymer particles is disclosed in U.S. Pat. No. 3,432,483. The process comprises the sequential steps of feeding to the polymer, water and a water-soluble block copolymer of ethylene oxide and propylene oxide surfactant into a dispersion zone; vigorously agitating the mixture under elevated temperature and pressure to form a dispersion of the molten polymer; withdrawing a portion of the dispersion and cooling to a temperature below the melting point of said polymer to form solid, finely divided polymer particles in the dispersion; reducing the pressure of said cooled dispersion to atmospheric pressure; separating the solid polymer particles from the surfactant solution phase and washing; drying the washed polymer particles; and recovering dry, finely divided powder.
While it is possible to produce fine powders of a broad spectrum of polyolefin polymers utilizing the above-described procedures, dispersion processes cannot be used to produce fine powders of resins with melt indexes below about 1. In the practice of the dispersion procedures, as the melt index of the resins being used approaches 1, it becomes increasingly difficult to achieve the type of dispersion necessary to form fine powders. Dispersions having droplets of the size necessary for the production of fine powders cannot be formed with fractional melt flow rate resins, i.e., resins having a melt index less than 1. This is believed to be due, in part, to the high molecular weights of such resins. The relationship of melt flow rate to molecular weight and the inability to form dispersions suitable for the production of fine powders with high melt flow rate resins is discussed in U.S. Pat. No. 3,746,681.
In view of the desirable physical properties of low melt flow rate resins, it would be highly advantageous if fine powders of such resins could be produced. It would be particularly advantageous if these fine low melt flow rate resin powders could be produced utilizing a dispersion process and if the particles had a relatively narrow particle size distribution and were spherical in shape. Coatings obtained using such powders would be expected to have improved thermal stability, improved creep resistance, improved chemical resistance and other desirable properties. These and other advantages are achieved by the present invention wherein fractional melt flow rate olefin copolymer powders are produced.
Ethylene/vinylalkoxysilane copolymers are described in U.S. Pat. Nos. 3,225,018 and 3,392,156. It is also disclosed in U.S. Pat. No. 3,392,156 that ethylene/vinyltrialkoxysilane copolymers can be used in finely divided form where the copolymer has an average size of less than about 10 mesh and preferably in the range of about 150 to 2000 microns. While the reference states that the finely divided material may be prepared by mechanical grinding, solution or dispersion techniques or other methods, no details are provided and no powders are prepared. Furthermore, it is a requirement of the process that the products be mechanically worked to obtain a reduction of melt index and an increase in stress cracking resistance. Melt indexes obtained after mechanical working range from 7.95 to zero in the examples.